Shielding arrangement



Dec. 23, 1930.

G. CRISSON SHIELDING ARRANGEMENT Filed Nov. 20, 1929 INVENTOR Y VQILATTORNEY Patented Dec. 23, 1930 UNITED sr'rss PATENT OFFICE GEORGECRISSON, OF EAST ORANGE, NEWiJERSEY, ASSIGNOR- TO AMERICAN TELEPHONE ANDTELEGRAPH COMPANY, A CORPORATION OF NEW YORK SHIELDING ARRAN GEMEN TApplication filed November 20, 1929. Serial N'o. 408,5'81.

This invention relates to shielding arrangements, and more particularlyto shielding arrangements to be used where cable conductors enclosed ina conducting sheath are connected to an amplifier and there is apotential difference between the ground at the amplifier and .overcomethis difficulty by providing two shields between trie windings of thetransformer, the one shield being connected to the office ground and theother shield heilig connected to the cable armor. Such constructiongives minimum potentials between each winding and its correspondingshield. Also, the potential difference between the office ground and thecable armor appears between the two shields, thereby causing a chargingcurrent through` the capacity between the shields, the path of whichcharging current does not include any part of either winding of thetransformer.

The invention will now be more clearly understood from the followingdescription when read in connection with the accompanying drawing thefigure of which is a diagram illustrating schematically apreferredembodiment of the invention.

Referring to the drawing, X designates a submarine or other type ofcable whose conductors l and 2 are enclosed, as is customary, in ametallic sheath 3 which rests upon the ground beneath the ocean and isin contact with the sea water. The conductors l and 2 of the cable areconnected to the input or grid circuit of a receiving amplifier Tthrough a transformer comprising a primary l? and a secondary S. Due tothe fact that the sheath of the cable is at a different potential fromthe office ground to which the input circuit of the amplifier isconnected, a noisy condition will result. The cable conductors, beingenclosed in the sheath, will have a much smaller potential difference tothe sheath than to the office ground, and consequently displacementcurrents will flow through the capacity between the windings of thetransformer from the ground of the cable sheath to the office ground.Since the path of these displacement currents will include at least apart of the windings ofthe transformer, noise potentials areproducedupon the grid of the amplifier.

It is proposed to overcome this difficulty, in accordance with thepresent invention, by providing two shields A and B between the windingsof the transformer, the shield A being connected to the office groundand the shield B being connected to the sheath 3 of 'the cable X. Withthis arrangement the potential difference between the sheath and theofhce ground causes displacement currents to flow through the capacitybetween the shields A and B, and these displacement currents flow fromthe sheath to the office ground over a path which does not include anypart of either the primary or secondary windings of the transformer.Owing to the large impedance of the path between the shields A and B,displacement currents flowing in the sheath 8 are very small and do notinduce large noise currents in the transmission circuit including theconductors l and 2 of the cable.. rlhe shield A acts as a short circuitto ground for the displacement currents to prevent them from flowingthrough the secondary winding S of the transformer, and the shield Blikewise acts to furnish a path for the displacement currentsindependent of the windings of the primary coil P of the transformer.

lf the shield A were omitted, leaving the shield B connected to thesheath 3 and unconnected to the ofiice rground at G, the displacementcurrents would flow from the sheath to the office ground through a partof the secondary winding S, due to the capacity between the shield B andthe windings of the secondary coil. f This would obviously result inproducing noise potentials upon the grid of the amplifier T, therebymaking the amplifier noisy.

Since the cable conductors l and 2 are enclosed in the sheath 3 theyhave a much smaller potential difference to the sheath than eX- istsbetween said conductors and the office ground at Gr. Consequently, ifthe shield B were omitted, using merely the shield A connected to theoffice ground, the primary winding I) would lie-exposed to the potentialvdiff erence between the vofiice ground and the cable conductors, andunless the primary P and the cable conductors were perfectly 'balancedwith respect to ground Y a noise lpotential would be set up between theconductors which would be repeated through the transformer to .theamplifier.

If, on the other hand, a single shield were used between the primary Pand the secondary S, and this shield were connected both to the oihceground at G and to the cable sheath 3, relatively large currents wouldiiow lthrough the cable sheath 3 and directly to ground at Gr. Due tothese relatively large currents flowing in the sheath noise currentswould be induced in the cable conductors which would be transferredthrough the transformer to the grid circuit of the amplifier. Inaccordance with the present invention, the large impedance due to theopen circuit between the shields A and B cuts down the currents in thesheath to the relatively small displacement currents which are able toflow through the capacity between the shields A and B, and therebyreduces the noise induced in the cable conductors. At the same time thetwo shields A and B provide a path for these relatively smalldisplacement currents which does not include any part of either windingof the transformer, the shields also protecting each winding from largepotentials.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

l. In a signaling system, a translating means having its input circuitassociated with ground, a transmission path inductively related to aground having a potential different from that associated with thetranslating means, a transformer for connecting said path to saidtranslating means, and conductively separate shields for saidtransformer, one connected to the ground associated with saidtranslating device and the other connected directly to the groundassociated with said transmitting path.

2. In a signaling system, a translating means having its input circuitassociated with ground, a transmission path having its conductorsenclosed within a conducting sheath, said sheath being at a groundpotential different from that associated with the input circuit of saidtranslating means, a transformer for connecting said path to saidtranslating means, and conductively separate shields for saidtransformer, one connected to the ground associated with saidtranslating device and the other connected directly to the ground ofsaid sheath.

3. In a signaling system, a v cur-.m tube `having an input circuitconnected to its grid and filament and its filament connected to ground,a transmission path having its conductors included within a conductingsheath, said sheath being at a ground potential different from that ofthe ground connected to t-he filament of said vacuum tube, a transformerconnecting said path to the input circuit of said vacuum tube, andconductively separate shields for said transformer, one connected to theground associated with the filament of said tube and the other connecteddirectly to the ground of said sheath.

4. In a signaling system, a vacuum tube having an input circuitconnected to its grid and filament and its filament connected to ground,a cable having its conductors enclosed within a conduct-ing sheath, saidsheath being connected to a ground potential different from that of theground connected to the filament of said vacuum tube, a transformerbetween the cable and said vacuum tube having a primary windingassociated with the cable and a secondary winding associated with theinput circuit of said vacuum tube, a shield for said primary windingconnected directly to the sheath of said cable, and a conductivelyseparate shield for the secondary winding of said transformer connectedto the ground associated with the filament of the vacuum tube.

In testimony whereof, I have signed my name to this specification this14th day of November, 1929.

GEORGE CRISSON.

